1. Field of the Invention
The present invention relates to an optically scanning displacement detector for determination of an object profile, and more particularly to such an optically scanning displacement detector utilizing a scanning unit for moving, in a scanning direction, a set of a light projecting unit for radiating a light beam on an object and a light receiving unit for receiving a reflected light from the object, to scan the light beam across the object.
2. Disclosure of the Prior Art
For determining an object profile by means of an optical measurement, U.S. Pat. No. 4,864,147 discloses an optically scanning displacement sensor 1P utilizing a triangulation. As shown in FIG. 35, the sensor 1P comprises a light projecting unit 2P for radiating a light beam on an object OBJ to be detected, a light receiving unit 3P for receiving diffuse reflected light from the object to generate an output position signal, and a distance determining unit (not shown) for determining a perpendicular distance between the object OBJ and the projecting unit 2P in accordance with the position signal. The light projecting unit 2P is provided with a light emitting element 10P such as a semiconductor laser, light emitting diode or the like, a first focusing lens 11P, and a scanning mirror 12P for deviating the light beam provided from the light emitting element 10P to scan the light beam across the object OBJ. The light receiving unit 3P is provided with a second focusing lens 21P and one-dimensional position detecting element 22P such as PSD.
However, there cause the following problems in case of scanning the light beam across the object OBJ having a cross-section shown in FIG. 36 by the use of the scanning mirror 12P.
1) Since the light beam is deviated at a deviating point M on the scanning mirror 12P to scan the light beam across the object OBJ the deviated light beam is incident on the object's surface at an inclined angle except for a specific position R of the object's surface positioned just below the deviating point M. Therefore, there is a first problem of forming a dead space D within a scanning range, in which the deviated light beam can not be radiated, as shown in FIG. 36. As an incident angle .phi.1 of the deviated light beam to the object's surface is smaller, the formation of dead space D is increased. As a result, accurate determination of the object's profile would be prevented by the dead space D. PA1 2) When there are two-different positions (R and P1) to be detected on the object's surface, which are spaced away from the scanning mirror 12P by a perpendicular distance d, as shown in FIG. 36, there causes a second problem that an area of a first light spot formed when the light beam is incident on the position P1 at an inclined angle .phi.2 is different from the area of a second light spot formed when the light beam is incident on the position R at a right angle .phi.3. The area difference between the first and second light spots is increased in proportion to a difference between the angles .phi.2 and .phi.3. Therefore, as a scanning range is increased, it would be difficult to determine the object's profile with a constant accuracy of measurement. PA1 3) The light receiving unit 3P uses such one-dimensional position detecting element 22P on which the reflected light makes a condensed light spot through the second focusing lens 21P. The detecting element 20P is disposed in the receiving unit 3P such the condensed light spot can be moved in a longitudinal direction Q of the detecting element 22P from a position S1 to a position S3 as a distance between the light projecting unit 2P and the object OBJ varies from a distance d1 to a distance d3, as shown in FIG. 37. When the light beam is scanned in a scanning direction across the object by the scanning mirror 12P, the condensed light spot moves on the detecting element 22P in a perpendicular direction to the longitudinal direction Q. Therefore, there is a third problem that a maximum scanning range of the displacement sensor 1P is limited by an effective width of the detecting element 22P. PA1 1) The scanning unit can provide a parallel movement of the light beam in the scanning direction to scan the light beam across the object. In particular, when the light beam is radiated from a substantially perpendicular direction to the object, a formation of the dead space can be effectively prevented. PA1 2) Since the carrier mounting the light projecting and receiving units is moved in the scanning direction to scan the light beam across the object, it is possible to perform a scanning operation in spite of size of the object. That is, the scanning unit can provide a wide scanning range. PA1 3) When there are two-different positions to be detected on a flat surface of the object which are spaced away from the carrier by a distance, an area of a first light spot formed when the light beam is incident on one of the positions is the substantially same as the area of a second light spot formed when the light beam is incident on the other position. It means that the displacement detector can provide improved accuracy of measurement, as compared with the case of using a scanning mirror for deviating a light beam to scan the light beam across the object.